Back testing is basically the process of checking our strategy against historical data.
MT4 is great for this but it relies on broker having good historical data and good data is basically non existent in MT4 and MT5.
However, don't let this discourage you, just be aware of the problem and make sure you update the history file of the currency pairs you are testing.
By the way, having updated the history file does not mean that the MT4 bars are correct. Often they will have wrong pricing or wrong time stamps so you should lower your expectations and don't trust what you see on the chart but they are good enough for manual back testing.
I don't recommend automated back testing of a strategy (using EA's) because the results will most likely be unreliable and unless a trader is very very experienced with MT4 and the particular strategy he is testing, he is not going to be able to understand and analyze the results and he could be made to believe that a strategy is profitable when in most cases it is not going to be profitable.....